Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!bloom-beacon!mit-eddie!rutgers!uwvax!speedy!stuart
From: stuart@speedy.cs.wisc.edu (Stuart Friedberg)
Newsgroups: comp.arch
Subject: "Compatible" (was Re: The NeXT Problem)
Summary: risc mc68020, ho-ho-ho
Message-ID: <6496@spool.cs.wisc.edu>
Date: 19 Oct 88 02:01:52 GMT
References: <26435@ucbvax.BERKELEY.EDU> <5498@juniper.uucp> <5941@winchester.mips.COM> <737@jetsun.WEITEK.COM>
Sender: news@spool.cs.wisc.edu
Reply-To: stuart@speedy.cs.wisc.edu (Stuart Friedberg)
Organization: U of Wisconsin CS Dept
Lines: 41

Once upon a time, ...

In article <5498@juniper.uucp> Christopher Michael Whatley writes:
>They are supposedly developing their own RISC chip that is compatible with the
>030. I don't know any more than that. I read this in a rumour column. 

In response, ...

In article <5941@winchester.mips.COM> John Mashey writes:
> This defies all logic.
> a) If it's compatible with an 030, it's not a RISC.

I agree with John, completely.

In article <737@jetsun.WEITEK.COM> Krishnan Sridhar writes:
>  Not quite true.  A RISC version of 68K can be made and with
>  appropriate compiler, can be made fully compatibile with their ancestors.

By Krishnan's argument, all we need to make the 6502 "fully compatible"
with a 370 is "an appropriate compiler."   I guess Krishnan means they
can run the same portable C programs.  But that's NOT fully compatible.

If the rumored CPU's instruction set (*I*nstruction *S*et is fully half of
the acronyms RISC and CISC) is not a strict superset of the MC680X0's
instruction set, then the rumored CPU is not fully compatible with the
MC680X0.  By any sane usage of the term "RISC", a strict superset of
the MC680X0's instruction set would NOT be a RISC instruction set.
Ergo, I agree with John, it defies all logic to say such a thing.
(However, marketing people do it all the time!)

If we accept *software* compatibility as our goal, then we don't have
to care very much at all about hardware compatibility.  Please witness
the orderly migration from Sun-2's and Sun-3's (based on MC680X0) to
Sun-4's (based on SPARC).  The SPARC is NOT fully compatible with a
MC680X0.  But none of the old portable software cares one whit. (Some
of the new software has new bugs, but that's not the processor's fault.)

Frankly, the rumor was stupid, should have been dismissed out of hand,
and certainly was not deserving of this much attention.

Stu Friedberg  (stuart@cs.wisc.edu)